1. Field of Invention
The present invention relates to switched capacitor arrays for use in DC-DC converters and more particularly to such arrays which are configurable to provide both buck and boost conversion.
2. Related Art
In electronic circuits, when multiple devices are to be powered by a single power supply, the power supply, operating on a single voltage source, must be able to provide a plurality of supply voltages to the various devices. In other applications, when a constant voltage is required by an electronic device, the power supply must be able to provide a regulated supply voltage from a variable voltage source, such as a battery. In both situations, a DC-DC converter utilizing a switched capacitor array is commonly used to provide the desired supply voltages. A switched capacitor array is generally a circuit of switches and capacitors which are configurable to provide multiple gains. A control unit for turning the switches on and off allows the capacitors to be configured and reconfigured such that select capacitors are charged and discharged to convert an input voltage to a desired output voltage, with the desired gain.
One common type of switched capacitor array is a voltage divider. The voltage divider circuit functions as a DC-DC converter to produce an output voltage which is lower than or equal to the value of the input or primary voltage source. Typical voltage divider arrays use N capacitors to produce different step-down or buck gains (gains .ltoreq.1) by configuring the capacitors in series across a voltage source to charge the capacitors and then configuring the capacitors in parallel to discharge the capacitors into a load capacitor. By alternately charging and discharging capacitors, gains within the gain region of 1/N are possible, i.e., 1/2, 1/3, . . . , 1/N. Even though such circuits have high operating efficiency, they are limited in that the output voltages are confined to integer divisions of the input voltage.
To increase the range of gains, U.S. Pat. No. 4,451,743, entitled "DC-to-DC Voltage Converter" to Suzuki et al., discloses a voltage divider circuit utilizing N capacitors and one main capacitor, which is incorporated by reference in its entirety. In Suzuki, the N capacitors are connected in parallel with one another and in series with the main capacitor during a charging cycle in which all the capacitors are charged. The N capacitors are then connected in series with one another and in parallel with the main capacitor during a discharging cycle in which the capacitors are discharged. As a result, a voltage divider is obtained which allows gains within the gain region of N/(N+1), i.e., 1/2, 2/3, 3/4, . . . , N/(N+1).
Another type of switched capacitor array is a voltage multiplier, which provides an output supply voltage greater than the input source voltage. Step-up or boost gains (gains &gt;1) are commonly achieved by configuring N capacitors in parallel with a battery during the charge cycle and in series across a load. capacitor during the discharge cycle. Similar to the voltage divider discussed above, such voltage multipliers, which are well known in the art, are limited to gain in the gain region of N, i.e., integer gains of 2, 3, . . . , N.
A voltage multiplier capable of providing non-integer gains is also disclosed in Suzuki et al., incorporated by reference above. The voltage multiplier in Suzuki utilizes N auxiliary capacitors and a main capacitor. The N auxiliary capacitors are configured in series with each other and in parallel with the main capacitor and a battery during the charging cycle to charge the N auxiliary and one main capacitor. During the discharge cycle, the N auxiliary capacitors are configured in parallel with each other and in series with the main capacitor for discharging into a load capacitor. As a result, a voltage multiplier is obtained which allows gains within the gain region of (N+1)/N, i.e., 2, 3/2, 4/3, . . . , (N+1)/N.
Although capable of non-integer voltage division or multiplication, the above-described switched capacitor arrays are incapable of providing both buck and boost conversion, which is desired to increase flexibility and performance in power supplies. For example, different output voltages or a wider range of operating input voltages may be desired. In such cases, a buck-only converter will be unable to supply the necessary power when the input voltage is less than the desired output voltage, and the efficiency of a boost-only converter will be reduced if the input voltage is greater than the desired output voltage.
In Suzuki, incorporated above, a separate non-integer voltage divider circuit and a separate non-integer voltage multiplier circuit are used in a DC-to-DC converter for providing both boost and buck conversions, with each circuit operating independently of the other. However, requiring separate boost and buck arrays necessarily increases the size of the DC-to-DC converter. Another type of buck and boost converter is disclosed in U.S. Pat. No. 5,414,614, entitled "Dynamically Configurable Switched Capacitor Power Supply and Method" to Fette et al., which is incorporated by reference herein in its entirety. In contrast to Suzuki, the switched capacitor arrays in Fette are each capable of providing either integer division or integer multiplication, with each array operating jointly for non-integer buck or boost conversion. However, similar to Suzuki, Fette requires two separate switched capacitor arrays to implement such a converter.
Accordingly, a switched capacitor array is desired which is configurable for both buck and boost conversion, resulting in a smaller and less complex voltage converter.